Soft X-ray Material Instrument

The SXR beamline will provide intense ultra short soft x-ray pulses generated by the free electron laser (FEL) with a highly diverse set of experimental configurations using established and powerful tools such as x-ray emission, coherent imaging, resonant scattering, photoelectron spectroscopy and x-ray absorption spectroscopy. The science that can be performed at the SXR beamline covers wide-spread fields such as catalysis, magnetism, correlated materials, clusters and biological structure.

The beamline is equipped with a monochromator whose energy range 500eV - 2000eV (initial operations will be at < 800 eV) covers several of the important K- and L-edges of the second and third row elements for resonant excitation with a resolving power in the order of 5000, but the monochromator can also deliver the full beam in non-monochromatic mode. A simple stationary experimental setup is provided in Hutch 1 (shared with AMO) before the monochromator where samples can be studied by XAS in transmission mode, detected in a single shot setup at the monochromator exit slit position.

The main interaction point situated in Hutch 2, after the monochromator and a set of K-B mirrors that focus the beam down to < 10x10um, is different with respect to other LCLS instruments in that it has no stationary end station. The consortium and various collaborators will role-up and connect different endstations and detectors to the instrument for the experimental program that can also be utilized by the general users via collaborations.

The soft x-ray imaging and pump-probe x-ray spectroscopy program on materials was approved by the LCLS Scientific Advisory Committee (SAC) in 2006, and space was allocated in the LCLS near hall for the accompanying instruments. A consortium was formed in order to fund, design and construct the SXR beamline with members from the Stanford Institute of Material and Energy Sciences (SIMES), the Advanced Light Source (ALS), the University of Hamburg, DESY and the Center for Free Electron Lasers (CFEL) in Hamburg. The consortium will serve as a central hub for a broader scientific community with interest in utilizing the unique capabilities of the LCLS in the soft x-ray regime. The consortium and various collaborators will also bring different endstations and detectors to the instrument for the experimental program that can also be utilized by the general users via collaborations. The Technical Design Report has been approved by LCLS and the SXR Consortium and LCLS has entered a Memorandum of Understanding (MoU) for the design, installation and operation of the SXR beamline.

Near Experimental Hall, Hutch 2 » complete instrument map

Pump-Probe Ultrafast Chemistry

The ultimate goal in chemistry and physical chemistry is to understand on a fundamental level how bonds break and reform during chemical reactions. In many cases we arrive at simple pictures of electron motion with respect to electron pair redistributions or electrostatic interactions along a reaction path. For many systems bonding can be understood in terms of molecular orbitals and reactivity in dynamical rearrangements of different molecular states. Such knowledge provides the basis for the understanding of chemical trends and prediction of chemical reactivity for chemical compounds. Since the excitation and probe steps with conventional optical lasers involve valence electrons that are delocalized over many atomic centers it is difficult to study complex systems. Unprecedented insight into chemical reaction dynamics would be gained by probing exactly the atomic site involved. X-ray spectroscopies can directly access molecular orbital changes associated with or even during chemical reactions. In particular, accessing core levels in the soft x-ray regime with spectroscopy opens up new prospects to study time-resolved changes in the electronic structure of complex systems containing the essential elements C, O and N or 3d-metal atoms. Detailed insight into surface reactions, catalysis, hydrogen-bonded systems and aqueous solutions can be expected.

X-ray spectroscopy has the unique ability to provide an atom-specific probe of the electronic structure. In x-ray emission spectroscopy (XES) the atomic or elemental sensitivity arises from the filling of a core hole by valence electrons from the same atomic site. In addition, core-level energy shifts (often denoted chemical shifts) connected with different environments allow for selective probing of chemically non-equivalent atoms (Figue 0-1). The final state of the x-ray emission process is a valence-hole state similar to the final state in valence band photoemission with the unique feature that the valence electronic structure is projected onto a specific atom. Notably, selection rules of XES, and similarly of X-ray photoelectron spectroscopy (XPS), in conjunction with variation of polarization vector of the incident light or angle-resolved detection of electrons allows to access molecular orbital symmetry and associated bond geometry. In addition, XPS can be uniquely tuned to high surface sensitivity, which is particularly desirable when studying interfaces, including the aqueous/vacuum interface. Resonant excitation and Auger electron spectroscopy gives unique access to the electronic structure of atoms and molecules in the gas phase, on surfaces and in liquids and solids. Some of the projects planned here are described in further detail in the following:

Surface Reactions and Catalysis: The microscopic understanding of heterogeneous catalysis requires a detailed understanding of the dynamics of elementary processes at surfaces including adsorption, formation of different intermediates, and desorption. These can be initiated by an ultrashort (optical) laser pulse, and the evolving product can be uniquely probed with XES and XPS using FEL soft x-ray pulses at a given time delay. Charge and energy transfer processes, for instance between an adsorbate and the (catalytic) substrate, can thus be studied with high site-selectivity and high temporal resolution. Likewise, we expect to identify and to characterize chemical bonding in short-lived reaction intermediates by transient changes of the electronic structure, from which kinetic models with an unprecedented level of detail can be derived.

Hydrogen Bonding, Radiation and Aqueous Solution Chemistry: Water is the key species for our existence on earth, and it is involved in nearly all biological, geological, and chemical processes. Knowledge about the hydrogen-bonded network structure in liquid water is essential for understanding its unusual chemical and physical properties. Infrared and optical excitations will be used to induce changes in the hydrogen bonding network, and to initiate reactions of hydrated molecules. Moreover, on a pratical level, radiation damage of concentrated electrolyte solutions, relevant for fuel storage, involves the anions interacting with ionizing radiation. To give another example, low-energy electrons created upon laser irradiation may attach to DNA bases in aqueous environment. Dissociative electron attachment is a major cause of strand breaks in DNA. Hydrogen-bond dynamics, solute-solvent interactions, and the interplay of electronic and nuclear dynamics during chemical reactions in solution will be accessible with time-resolved optical pump and x-ray probe spectroscopy. Techniques include XAS, XES, and XPS measurements; the latter will be performed in conjunction with the liquid microjet technique.

Warm Dense Matter: Warm Dense Matter refers to the region of the density-temperature phase-space between solids and plasmas, where the standard theories of condensed matter physics and/or plasma statistical physics are invalid. [Ao el al, PRL 96 (2006) 55001] These states of matter are of broad interest, as similar conditions of high temperatures and pressures exist in planetary interiors and in shock compressed matter. Further, there is an incomplete understanding of how materials damage under Free Electron Laser (FEL) irradiation. A femtosecond optical or FEL pulse will isochorically heat the sample. The x-ray emission will yield information about the occupied density of states. The objective is to develop a quantitative understanding of the electronic structure of materials at solid densities and temperatures of several 1000 K.

Clusters as new Materials

Materials assembled atom by atom offer the chance to create new materials with controlled but as yet unprecedented new properties. In the nanometer size range, new electronic, optical, chemical, magnetic or even mechanical properties arise due to the effects of the quantized nature of the electronic interactions. Thus, as far as the development of the novel materials is concerned, the properties of these particles are not predictable by scaling laws. In this size range every atom counts, i.e. adding or removing just a single atom from the particle will result in different materials properties. One of the most prominent examples of this kind are the materials formed by condensing C60 or related fullerenes into solids bound by van der Waals forces. These purely carbon based solids exhibit distinctly different properties from diamond or graphite, the other known solid carbon materials.

LCLS offers the unique opportunity to study both the electronic properties as well as the actual atomic structure of individual mass selected particles with an exactly defined number of atomic constituents. The pulse properties of the LCLS source are required for these studies, since cluster production coupled with mass selection results in a highly diluted target density. The electronic properties will be accessible by studying the Auger spectrum following the creation of a core hole, whereas the atomic arrangement will be studied by core level photoemission (XPS) and (N)EXAFS, once the photon energy at LCLS can be readily tuned. For non-resonant Auger spectroscopy the exact photon energy does not need to be determined, as long as it is sufficiently high above the core electron absorption threshold. For XPS the monochromator running in spectrograph mode is required, since the photon energy and photon lineshape need to be recorded for each LCLS pulse individually together with the XPS spectrum.

Control of the magnetic properties of nanoparticles:The control of the properties of magnetic nanoparticles is crucial for applications ranging from to data storage to cancer treatment. Under basic science aspects, it is intriguing to note that all atoms, except the rare gases, exhibit magnetism (Hund's rule). Thus, in the form of well defined clusters, even elements such as Al form magnetic particles. As extended solids, on the contrary, only the latter part of the transition metals and the f-electron systems are magnetic. In general, the magnetic moments of clusters are larger than in the corresponding solids, whereas the temperature, below which magnetic order is stabilized, is lower. In summary, there are many degrees of freedom to design the magnetic properties of nanoparticles by controlling the exact size and composition, whereby not only metals, but also oxides are interesting candidates.

Control of the chemical properties of nanoparticles: Not only transition metals and their oxides are interesting candidates as catalysts, but also small Au and Ag clusters and their oxides exhibit quite a high catalytic activity, for example in various oxidation reactions. Again, the electronic structure of individual mass selected nanoparticles will be probed by (resonant) Auger spectroscopy and XPS to elucidate not only the particle properties but also the chemical bonding of adsorbates, while pump-probe spectroscopy will give insight into photo-chemical reaction cycles.

Magnetic Imaging

One of important topics of physics is the study of phase transitions, where structural, electronic, or magnetic properties undergo discontinuous or continuous changes. Powerful symmetry and statistical concepts have been developed to describe such phase transitions, and corresponding scattering and thermodynamic measurements have been used for experimental verifications. Such approaches are quite successful and satisfactory to many. Complementary and equally powerful, the conceptualization of a simple physical picture in the real space has played an important role in the formulation of physical understanding of many phenomena. This method of direct representation, however, has not received equal recognition perhaps due to the lack of corresponding experimental techniques until now. By combining the powerful Fourier Transform Holography (FTH) imaging technique with LCLS' high brightness, short pulse structure, and fully transverse coherence, the dynamics of magnetic fluctuations and magnetization relaxation processes can be visualization at extremely fast time scales and at nm resolutions. This soon-to-be-established new capabilities will not only provide direct experimental proof of the symmetry and statistical concepts in magnetic phase transitions, but will also have far-reaching impact beyond magnetism in the studies of critical phenomenon such as other order/disorder transitions or the demixing of binary alloys.

The proposed experiments will de designed to investigate the critical fluctuations occurring at magnetic phase transitions. An initial LCLS experiment will demonstrate the existence of magnetic spin blocks by taking snap shot pictures on a time scale faster than the fluctuations. The next step will be to study the spin block dynamics by recording a series of such pictures at well-defined delay times. For imaging of the magnetic spin blocks a newly developed lensless imaging technique will be used. These experiments, however, would not be possible without the LCLS' ultra-bright, ultra-short, and fully coherent x-ray pulses. Key properties and implications of the proposed experiment are:

Single x-ray pulse imaging: A single LCLS pulse will be sufficient to obtain an image of the instantaneous magnetic domain structure which is expected to be static on the time scale set by the ultra-short x-ray pulse length (230 fs), even close to the phase transition.

50 nm spatial resolution or better: It has been demonstrated that a 50 nm spatial resolution can be achieved with the newly developed lensless x-ray imaging technique which will take full advantage of the unprecedented full coherence of LCLS x-ray pulses. Subsequent phase retrieval may improve this to near-wavelength limited spatial resolution.

Dynamics of critical fluctuations: To image the fluctuation dynamics we will split the beam and produce consecutive x-ray pulses with a well defined time separation at the sample. From each pulse we will obtain an image of the magnetic domain structure, using a scheme discussed below, and thus resolve the dynamics occurring on a femtosecond to nanosecond time scale.

Ultrafast, non-deterministic dynamics: It is important to realize that the proposed experiments significantly differ from today's ultrafast pump-probe experiments. Such experiments rely on reversibility of the sample to a well defined state before each pump-probe cycle. In contrast, the critical fluctuations that are the subject of our study are non-deterministic and their study requires complete images to be recorded in a single shot.

Ultrafast relaxation dynamics: It is clear that once we have demonstrated the feasibility of ultrafast single shot imaging a whole class of new experiments will become possible. Besides single shot imaging of spontaneously occurring fluctuations we also envision to study ultrafast relaxation dynamics in pump-probe experiments.

X-Ray Scattering Spectroscopy on Strongly Correlated Materials

Among the current research issues of condensed matter physics, the electronic structure of the strongly correlated materials is one of the most active topics. In this class of materials, the Coulomb interaction between electrons can not be ignored, which manifests itself as 'strong correlations' acting as a 'tuning parameter' to switch the ground state from one to the other. These collective ground states for different phases are known as 'emergent' phenomena of many body systems, which can not be deduced from any perturbation theory and often involve novel forms of order.

X-Ray scattering experiments on strongly correlated materials have been performed to probe the charge ordering and elementary charge/magnetic excitations containing important information of the ground state properties. Recent theoretical developments show that resonance x-ray scattering can provide rich information on many-body wavefunctions. Soft x-ray, being sensitive to valence electrons and in the spectral range of important L edges of transition metals that often are key elements of correlated materials, provide special opportunity. However, so far most of these experiments were performed in the equilibrium state, which can not provide any information along the time axis regarding how the electrons form this particular ground state. The exciting opportunity provided by the SXR of LCLS is exactly this missing piece of information along the time axis. Using the high pulse intensity and ultra-short pulse length, it is possible to perform optical-pump-and-X-ray-probe experiments to study how the electronic states of strongly-correlated materials relax from an excited state to the ground state. The relaxation process is closely related to the correlation effect among the electrons and the electronic interactions to other degrees of freedom; therefore 'snapshots' obtained from the pump-probe experiments provide important clues to construct a microscopic physics picture of the strongly-correlated systems. In addition, x-ray probe experiment also has some unique advantages, such as element specific information, bulk sensitive signal, and the dynamic structure factors, which are not accessible by the most common ultrafast optical pump-probe experiments in the visible light regime.

Absorption experiment: Absorption spectrum reveals the partial density of states of the unoccupied state of the materials. As a first step, it is important to understand how density of state change after the system been 'pumped' by the optical Laser pulse. It is also an ideal initial experiment to do for the initial operation stage of the LCLS, since it is not an extremely photon hungry experiment, which can be done using with less powerful pulse and lower repetition rate. In addition, this absorption experiment would become a routine diagnosis experiment for the chamber alignment, sample damage assessment and a survey experiments for the resonant scattering experiments.

Resonant Elastic Scattering experiment: Charge, spin and orbital order is one of the interesting phenomena in many strongly correlated electron systems. These orderings, mostly incommensurate to the lattice constant, produce extra Bragg peaks in the elastic X-ray diffraction pattern. As these orders are often complex and thus relatively large in real space, making it possible for soft x-ray to have sufficient q to probe the extra Bragg peak. Using SXR pump-probe capability of LCLS, one could destroy the charge ordering by the optical pump Laser and probe by the LCLS X-ray pulse at different delay times after the pump as the system relaxes back to the charging ordering state. This experiment shall reveal important microscopic information of the charge ordering formation.

Resonant Inelastic X-ray Scattering (RIXS) Experiment: To probe the excitations of the ground state, it is necessary to record the energy loss of an inelastic scattering process. In addition, the information at different momentum transfers of the scattering process is also extremely important for the scattering experiments on solids as it is related to the momenta of these excitations. Time resolved pump-probe RIXS experiments can be used to measure time evolution of collective modes and thus, information about the dynamics involves in the many-body excitations that gave the collective modes. In addition, we will also use RIXS to obtain the wave function properties through various projections to its intermediate states. The pump-probe experiment using the SXR shall provide a description of the wave function evolution from the pumped exciting state to the ground state.

High-Resolution Ultrafast Coherent Imaging

X-ray microscopy at synchrotron sources is steadily progressing, with the nanofabrication of better zone-plate lenses and the development of lensless coherent imaging techniques. However, even with cryogenic sample cooling, radiation damage limits the achievable resolution to about 10 nm. Higher resolution is required to understand the structure and organization of living (unstained and unsectioned) cells, and would greatly complement real-time optical fluorescence microscopy to study cell processes, such as cell division, and full function of components such as the cytoskeleton. Coherent diffractive imaging with intense and ultrashort X-ray pulses could achieve the required resolution on living cells by recording the scattering information before any structural changes due to interaction with that pulse. This method of flash imaging has been verified at FLASH. In principle the resolution should only be limited by the wavelength of the radiation, given the appropriate pulse parameters and size of the focused pulse. The details of the matterFEL interaction must be studied to gain an understanding of achievable resolution limits. Methods must also be developed for 3D imaging of reproducible samples, using streams of particles in the gas phase or in droplets. These delivery systems, first developed at FLASH, are required to quickly replenish the sample and allow time-resolved stroboscopic imaging of laser alignment of particles. Also the coherent diffractive method (with fixed or injected samples) enables the highest spatial resolution of ultrafast processes in non-periodic systems, such as the study of laser-induced phase transitions in materials. Experiments at FLASH demonstrated this technique on the study of phase separation in laser-ablation, but the shorter wavelengths of LCLS are required to achieve the necessary spatial resolution.

Imaging of biological cells beyond radiation damage limits: With the X-ray intensity provided by 5-micron focusing it will be possible to achieve a single-shot resolution of better than 5 nm. Imaging will be carried out on cells on thin membranes that can be placed into the beam as well as on cells injected into the LCLS beam.

FEL-matter interactions: Initial experiments will study the effects of damage by collecting coherent patterns of homogeneous samples (such as polystyrene spheres or nanocrystals) as a function of pulse fluence. In this case, the scattering pattern immediately gives the pulse-integrated size distribution of the particles, which can be compared with theory. Nanocrystals of biomolecular complexes (such as PS1) will give the high-resolution information about the damage of organic material as a function of resolution and pulse fluence.

Time-Delay Holography: This method, tested at FLASH, will be implemented to measure movies of the interaction and evolution of reproducible samples (such as virus particles) with FEL pulses. The apparatus will include a reflecting crystal, such as InSb, to direct the pulse back onto the sample. The detector will be placed in a backscattering geometry. Samples will include layered spherical structures to test methodologies to prolong the onset of damage.

Laser-matter interactions: A synchronized optical pump pulse will be used for single-shot time-resolved imaging in materials and stroboscopic imaging of laser-particle interactions (such as laser alignment).